The use of an excimer laser as a light source of a reduction projection aligner for semiconductor device production attracts public attention. This is because many excellent advantages are expected: i.e., there is the probability that the limit of the resolution will be enhanced to less than 0.5 um since the wavelength of the excimer laser is short (the wavelength of a KrF laser is about 248.4 nm); the depth of the focus is great compared to the g-and i-lines from a mercury lamp used conventionally with the same resolution: the numerical aperture (NA) of a lens may be reduced; a large exposure area is available, high power is available, etc.
However, there are two big problems to be solved when the excimer laser is used as the light source of a reduction projection aligner.
One is that since the wavelength of the excimer laser is short, namely, 248.35 nm, the materials transparent to this wavelength are only quartz, CaF.sub.2 and MgF.sub.2, and only quartz can be used as the lens material among these named materials from a standpoint of uniformness and working accuracy. Therefore, it is impossible to design a reduction projection lens with corrected chromatic aberration. Thus, it is necessary to narrow the bandwidth of the excimer laser to such an extent that the chromatic aberration is negligible.
The other problem is how to suppress a speckle pattern produced by narrowing the excimer laser band and how to suppress a reduction of power produced by narrowing the band.
There is a technique of narrowing the excimer laser band called an injection locking system. This system includes wavelength selective elements (an etalon, a diffraction grating, a prism, etc.,) disposed within a cavity of an oscillator stage. The system is oscillated in a single mode by limiting a spatial mode using a pinhole, the laser beam is injection synchronized by an amplifier stage. Thus, the output beam is high in coherency. If this output beam is used as a light source for the reduction aligner, a speckle pattern will be generated. Generally, the generation of a speckle pattern is considered to be dependent on the number of spatial transverse modes contained in the laser beam. It is known that if the number of spatial transverse modes contained in the laser beam decreases, a speckle pattern becomes likely to be generated whereas if the number of spatial transverse modes increases, the speckle pattern becomes less likely to be generated. The injection locking system is a technique of narrowing the band essentially by greatly reducing the number of spatial transverse modes. However, the generation of a speckle pattern is a big problem, so that the injection locking system cannot be employed in a reduction projection aligner.
A promissing technique of narrowing the excimer laser band uses etalons which is a wavelength selective element. As a conventional technique using etalons, AT & T Bell Laboratories has proposed a technique of narrowing the excimer laser band by disposing etalons between a front mirror and a laser chamber of the excimer laser. However, there is the problem that in this system the spectral profile cannot be narrowed so greatly, and that power loss is large due to insertion of the etalon, and the drawback that the number of spatial transverse modes cannot be increased so greatly.
Inventors have succeeded in obtaining a laser beam having an output of about 50 mJ per pulse by uniformly narrowing the spectral width to within about 0.003 nm in full width at the half maximum over whole beam size of 20.times.10 mm in which an etalon having a large effective diameter (of about dozen millimeters) is disposed between the rear mirror and laser chamber of the excimer laser. By employing the structure in which the etalon is disposed between the rear mirror and the laser chamber of the excimer laser, essential problems required for a light source for a reduction projection aligner including narrowing the laser band, ensuring the number of spatial transverse modes, and reduction of the power loss due to insertion of the etalon have been solved.
Although the structure in which the etalon is disposed between the rear mirror and laser chamber of the excimer laser has excellent advantages such as narrowing the band, ensuring the number of spatial transverse modes, and reduction of the power loss, power transmitting through the etalon is very high, so that physical changes fluctuations in the etalon temperature will occur. Therefore, there is the problem that the central wavelength of the oscillated output laser beam may fluctuate and the power may be greatly reduced. This tendency is especially remarkable in the use of two or more etalons with different free spectral range for narrowing the band.
It is an object of the present invention to provide in a laser having a structure in which a wavelength selective elements are disposed between the rear mirror and the laser chamber of the laser a laser wavelength control apparatus in which the central wavelength of the output laser beam is locked with high accuracy to reduce fluctuations of the laser power to thereby provide a stabilized output.